1. Field of the Invention
The present invention relates to an angular acceleration sensor that detects angular acceleration using deflection stress generated in a beam and an acceleration sensor that detects acceleration using deflection stress generated in a beam.
2. Description of the Related Art
Angular acceleration sensors and acceleration sensors include a fixed portion, a weight, a beam, and a detector. The weight is elastically supported on the fixed portion by the beam. The detector detects the angular acceleration or the acceleration of the weight using stress generated in the beam.
Some angular acceleration sensors and acceleration sensors have a beam in which a through-hole is formed in order to reduce the rigidity of the beam (see, for example, Japanese Unexamined Patent Application Publication No. 2007-322200).
FIG. 8 is a plan view of an angular acceleration sensor having an existing structure according to Japanese Unexamined Patent Application Publication No. 2007-322200.
An angular acceleration sensor 101 includes a fixed portion 102 having a frame shape; a weight 103 having a planar shape; a beam 104 that forms a cross shape with the weight 103 and that is connected to the fixed portion 102 at both ends of the beam 104; and detection electrodes 105A, 105B, 105C, and 105D that are disposed on both sides of end portions of the beam 104. The beam 104 has through-holes 106, which are formed in the beam 104 excluding a part of the beam 104 connected to the weight 103. The through-holes 106 extend over substantially the entire length of the beam 104 and pass through the center of the beam 104 in the width direction.
By forming the through-holes 106 in the beam 104, the rigidity of the beam 104 is reduced. Therefore, the deflection of the beam 104 per unit angular acceleration of the weight 103 is increased. As a result, sensitivity in detecting angular acceleration is increased, and the S/N ratio of a detection signal is increased.
When a through-hole is formed in a beam, the rigidity of the beam is reduced and thereby the resonant frequency is reduced. It is difficult for acceleration sensors or angular acceleration sensors to detect a signal in a frequency band that is higher than a resonant frequency. Therefore, if the resonant frequency is low, a wide detection band cannot be realized. In order to increase the resonant frequency, it is necessary to change the design of the sensor so as to increase the width of the beam or decrease the width of the through-hole. However, if the design is changed in such a way, the rigidity of the beam is increased and the deflection of the beam is reduced, and therefore the sensitivity of the sensor is reduced. Accordingly, to date, it has been difficult to achieve both a wide detection band and a high S/N ratio.
If a housing or the like that supports the fixed portion becomes deflected or the temperature of the housing or the like becomes uneven, the fixed portion may receive stress and the stress may be transmitted from the fixed portion to the beam. If this occurs, the stress may be detected by the detector, and the S/N ratio of a detection signal may be reduced. In a case where the width of the beam is increased in order to obtain a wide detection band, the stress that the fixed portion receives is easily transmitted to the beam, and therefore the S/N ratio for a detection signal tends to be reduced. Thus, also in this respect, it has been difficult to achieve both a wide detection band and a high S/N ratio.